Conventionally, there have been various proposals regarding methods for manufacturing sulfonium salts with polyhalo-metal, semimetal or nonmetal anions. In all of these conventional manufacturing methods, a sulfoxide compound and a sulfide compound are first condensed in the presence of a strong acid such as sulfuric acid, polyphosphoric acid, methanesulfonic acid or the like so that a sulfonium salt of the strong acid used is formed; then, this reaction mixture is added to an aqueous solution of a polyhalo-metal, semimetal or nonmetal salt, i.e., KBF4, KPF6, KSbF6, KAsF6, sodium salts of the same or the like, in order for metathesis to take place, after which the desired sulfonium salt with polyhalo-metal, semimetal or nonmetal anion is recovered by filtration or the like.
Methods for forming such a sulfonium salt of a strong acid by condensing a sulfoxide compound and a sulfide compound in the presence of this strong acid include a method in which a sulfoxide compound and an aryl sulfide compound are subjected to dehydration condensation in sulfuric acid, thus forming an arylsulfonium sulfate or hydrogensulfate (see Japanese Patent Application Laid-Open No. S61-212554), a method in which a diaryl sulfide compound is partially oxidized, thus producing a mixture consisting of substantially equimolar amounts of the diaryl sulfide compound and corresponding diaryl sulfoxide, and this is then converted into a triaryl sulfonium salt using a strong acid such as sulfuric acid in the presence of a dehydrating agent such as acetic anhydride (see Japanese Patent Application Laid-Open No. S61-100557), a method in which a diaryl sulfoxide compound and a polyaryl sulfide compound are reacted in an alkylsulfonic acid (e.g., methanesulfonic acid) solution of acid anhydrides such as phosphorus penfoxide (see Japanese Patent Application Laid-Open No. H5-4996) or acetic anhydride (see Japanese Patent Application Laid-Open No. H7-82244), a method in which a diaryl sulfoxide compound and a polyaryl sulfide compound or the like are reacted in the presence of polyphosphoric acid (see Japanese Patent Application Laid-Open No. H7-82245), and the like.
However, a condensation reaction performed by the above-mentioned conventional methods, have drawbacks in yield and cost: sulfonation of aromatic rings occurs in the case of a reaction performed in sulfuric acid, so that there is a danger that the yield of the desired product will drop. Furthermore, depending on the type of aryl sulfide used, a bis-sulfonium salt may be produced even if an attempt is made to obtain a monosulfonium salt, as described in the comparative examples of the present specification: the reaction in an expensive alkylsulfonic acid such as methanesulfonic acid results in an increase in cost: side reactions occur in the case of a reaction performed in polyphosphoric acid, resulting in low yield of the desired product.
Furthermore, in all of the above-mentioned conventional methods, the reaction mixture of a sulfonium salt with the strong acid residue is added to an aqueous solution of an alkali metal polyhalo-metal, semimetal or nonmetal salt, in order for metathesis to take place. However, since polyhalo-metal, semimetal or nonmetal salts have a low solubility in water, a large amount of water must be used in order to prepare aqueous solutions of these salts. Accordingly, large quantities of waste water are generated after the desired sulfonium salt is recovered by filtration or the like. Furthermore, since this waste water contains strong acids or acetic anhydride used in excess in the reaction and is strongly acidic, it must be neutralized with caustic soda or the like before disposal, thus bringing about a problem that the amount of waste water is further increased.
The present inventors conducted diligent research in order to solve the problems encountered in conventional methods, such as a drop in yield caused by the occurrence of side reactions, the use of expensive acids, and generation of large quantities of waste water due to metathesis and neutralization. As a result, the inventors discovered a method which makes it possible to manufacture target sulfonium salts directly without using a large excess quantity of acid and without using a metathesis process. This discovery led to the present invention.